278 SYSTEMATIC PALEONTOLOGY 



umbonal ridge inconspicuous in a lateral view, rather sharply defined, 

 however, in a dorsal view by a narrow furrow which outlines a wide 

 lanceolate flattened area, equally divided by the hinge line, and in the 

 upper part of which (immediately behind the beaks) the ligament is 

 attached to distinct fulcra; anterior dorsal slope abruptly rounded; 

 entire anterior half of valves strongly ventricose, while between this part 

 and the posterior umbonal ridge a -slight sulcus crosses from near the 

 beak to the base. Surface marked by rather distinct, closely arranged, 

 subequal concentric striae of growth, tending to irregularity in the basal 

 parts of old shells. 



" Impressions of adductor muscles extremely deep, the anterior pair 

 larger than the posterior. A small, though distinct, pedal muscle scar is 

 always present on the upper part of the strong ridge which forms the 

 inner boundary of the anterior adductor (in casts it lies at the bottom of 

 the deep cavity produced by this ridge), but the corresponding posterior 

 scar is rarely distinguishable. Hinge plate very narrow at the beaks, but 

 widening rapidly on each side, the anterior half somewhat the stronger 

 and slightly concave along its inner margin, both terminating abruptly 

 at the muscular scars; denticles 12 behind and 10 or 11 in front, those 

 near the beaks very small, all interlocking deeply, especially those of the 

 anterior set, which are also somewhat larger than the posterior. The 

 shell is very thick and the rostral filling so considerable that in casts of 

 the interior the beaks appear obtuse and widely separated." Ulrich, 1894. 



A characteristic Black Eiver species of Canada, New York, and the 

 Ohio and Mississippi valleys.. 



Occurrence. CHAMBERSBUEG LIMESTONE (Tetradium cellulosum 

 bed). Fort London, Franklin County, Pennsylvania. 



Collections. Maryland Geological Survey, U. S. National Museum. 



CTENODONTA OBLIQUA Hall 

 Plate LIV, Figs. 17-19 



Nucula obliqua Hall, 1845, Amer. Jour. Sci., vol. xliii, p. 292. 

 Tellinomya ? oUiqua, Meek, 1873, Pal. Ohio, vol. i, p. 139, pi. xi, figs, lla-c. 

 Ctenodonta obliqua Ulrich, 1894, Geol. Minnesota, vol. iii, pt. 2, p. 604, 

 pi. xlii, figs. 83-87. 



